Video surveillance or security cameras are often used for monitoring locations inside or outside of buildings. These buildings include government, residential, and commercial buildings, such as hospitals, casinos, retail establishments (e.g., shopping malls and department stores), offices, or warehouses, to list a few examples. The video cameras typically capture video data as a series of images (or frames) and send the video data to a control system, workstation, and/or network video recorder (NVR). The video data may be monitored in real time by security personnel or stored in the network video recorder and viewed at a later time.
Wide dynamic range (WDR) cameras are often used in security systems because the video cameras many times operate in locations with varying lighting conditions and must image objects of widely different intensities. Dynamic range refers to the ratio between the maximum and minimum measurable light intensities. For example, video cameras installed to monitor a parking lot at night will see bright headlights and area/street lights and dark shadows at the edges.
Currently, wide dynamic range (WDR) cameras utilize a number of different technologies. For example, some WDR video cameras implement multi-frame imaging. In this method, the video cameras capture multiple images of the same scene with different exposure settings (e.g., iris setting and shutter speed). The different images are then combined into one image. Other cameras use logarithmic sensors, which have different sensitivity at different intensity levels. Because the logarithmic sensors are less sensitive to changes in bright objects and more sensitive to changes in darker objects, the sensors provide better contrast in the darker parts of a scene.
The video cameras use different automatic exposure modes (metering modes) to calculate the exposure settings for a scene. For example, one metering mode is average metering. In this mode, light from the entire scene is averaged to determine the exposure settings. Conversely, in spot metering, only light from a small focus area (typically 1-5%) or “spot” is used to determine the exposure settings. The light from other areas of the scene is not factored into the exposure settings. Yet another metering mode is center-weighted metering. In this mode, light from a center focus area is weighted significantly more than the light from areas outside of the center area.
Partial metering and multi-zone metering are two additional metering modes. Partial metering is similar to spot metering, but includes a larger focus area (typically 10-15%). Similar to spot metering, light outside of the focus area is not used to determine the exposure settings. Additionally, multi-zone (or matrix) metering uses light from multiple areas in the scene to determine exposure settings.